CN103187035B - System and method for adaptively controlling LED backlight - Google Patents

Abstract

The invention relates to a system and a method for adaptively controlling a light emitting diode backlight source. Which analyzes the brightness of the image data to be displayed on the display panel through a content analyzer. And controlling the illumination of the light emitting diode backlight source by the light emitting diode driver according to the analysis result of the content analyzer by using the light emitting diode current controller. When the analysis result of the content analyzer shows that the brightness of the image data is higher than a preset value, the light-emitting diode current controller overdrives (over-drive) the light-emitting diode backlight source, so that the driving current flowing through the light-emitting diode backlight source is larger than a normal current.

Description

System and method for adaptively controlling LED backlight

technical field

The invention relates to a backlight source, in particular to a system and method for adaptable control of a light-emitting diode (LED) backlight source suitable for a flat display panel.

Background technique

The backlight is disposed on the back or side of the flat display panel (such as the liquid crystal display panel) to provide light. The backlight source can be a cold cathode fluorescent lamp (CCFL), a light emitting diode (LED) or other light sources. The light emitting diode is generally used as a backlight source because of its advantages of low power consumption, fast response and long service life.

FIG. 1A is a schematic diagram of an LED backlight, which is composed of multiple strings of LEDs connected in parallel, and each string of LEDs is composed of multiple LEDs connected in series. As shown in FIG. 1B , FIG. 1B is a graph showing the relationship between the luminous intensity of the LED and the forward current. From the figure, it can be known that the luminous intensity of the LED is linearly proportional to the forward current.

Conventional backlights (such as LED backlights) generally have a fixed luminous intensity. Since the image data usually does not occupy the entire dynamic range (ie from the darkest to the brightest), the dynamic display range of the display panel cannot be effectively utilized. In addition, another disadvantage of conventional backlights (such as LED backlights) is their low dynamic contrast.

It can be seen that the above-mentioned existing light-emitting diode backlight obviously still has inconveniences and defects in product structure, method and use, and needs to be further improved urgently. In order to solve the above-mentioned problems, the relevant manufacturers have tried their best to find a solution, but no suitable design has been developed for a long time, and there is no suitable structure and method for general products and methods to solve the above-mentioned problems. This is obviously a problem that relevant industry players are eager to solve. Therefore, how to create a new system and method for adaptively controlling LED backlight, so that it not only has enhanced contrast, but also can maintain the advantages of LEDs, is one of the current important research and development topics, and has become the current industry. A target in great need of improvement.

Contents of the invention

The purpose of the present invention is to overcome the defects of the existing light-emitting diode backlight and propose a new system and method for adaptively controlling the light-emitting diode backlight. The technical problem to be solved is that it can be used to greatly enhance the display image The dynamic contrast ratio or/and save considerable energy, very suitable for practical use.

The purpose of the present invention and the solution to its technical problems are achieved by adopting the following technical solutions. According to the present invention, a system for adaptively controlling a light-emitting diode (LED) backlight source includes a content analyzer, an LED driver, an LED current controller, and a current limiter. Wherein, the content analyzer analyzes the brightness of the image data to be displayed on a display panel. The LED driver is used to drive the LED backlight. The LED current controller controls the illumination of the LED backlight through the LED driver according to the analysis result of the content analyzer. The current limiter is used to limit the driving current flowing through the LED backlight to an upper limit. When the analysis result of the content analyzer shows that the brightness of the image data is higher than a preset value, the LED current controller over-drives the LED backlight, so that the driving current flowing through the LED backlight is greater than one. normal current.

The purpose of the present invention and its technical problems can also be further realized by adopting the following technical measures.

In the aforementioned system for adaptively controlling LED backlight sources, the LED driver includes a pulse width modulation (PWM) controller for determining the duty cycle of a pulse width modulation signal, wherein the pulse width modulation The signal is coupled to control the driving current flowing through the LED backlight.

In the aforementioned system for adaptively controlling the LED backlight source, when the analysis result of the content analyzer shows that the brightness of the image data is lower than a preset value, the LED current controller under-drives the light emitting diode The LED backlight makes the driving current flowing through the LED backlight smaller than the normal current.

The aforementioned system for adaptively controlling the LED backlight source further includes: an overdrive timer for recording the overdrive time of the LED backlight source; and a temperature estimation unit based on the duty cycle of the PWM signal And record the over-driving time to estimate the temperature of the over-driven LED backlight; wherein, when the estimated temperature reaches a high temperature upper limit, then reduce the driving current.

The aforementioned system for adaptively controlling the LED backlight further includes a temperature sensor for detecting the temperature of the LED backlight that is over-driven, wherein when the detected temperature is higher than a preset temperature threshold value, reduce the drive current.

The aforementioned system for adaptively controlling LED backlights further includes a processor for performing the following steps: accumulating the driving current within a period to form an accumulated driving current value; according to the accumulated driving current value and the period, Obtain an average current value; and when the average current value is greater than an upper limit value, then reduce the drive current; wherein, repeat the step of reducing the drive current until the average current value is less than the upper limit value minus a hysteresis value .

The aforementioned system for adaptively controlling the LED backlight source also includes a processor for performing the following steps: using a first counter for counting to generate a first cumulative driving current for a unit period; using a second The counter is used for counting to generate the second cumulative driving current of the double unit period, wherein the first half of the double unit period coincides with the unit period; according to the second cumulative driving current and the double unit period, to obtain an average driving current; obtaining the difference between the second cumulative driving current and the first cumulative driving current as a new first cumulative driving current; and repeating the counting steps of the second counter to obtain a new double A new second cumulative driving current for the unit period, wherein the first half of the new double unit period coincides with the second half of the double unit period.

The purpose of the present invention and the solution to its technical problem also adopt the following technical solutions to achieve. A method for adaptively controlling LED backlight according to the present invention includes the following steps: analyzing the brightness of image data to be displayed on a display panel to generate an analysis result; controlling the LED backlight according to the analysis result source to drive the LED backlight; and limit the driving current flowing through the LED backlight to an upper limit; wherein, when the analysis result shows that the brightness of the image data is higher than a preset value, Then, the LED backlight is over-driven, so that the driving current flowing through the LED backlight is larger than a normal current.

The purpose of the present invention and its technical problems can also be further realized by adopting the following technical measures.

In the aforementioned method for adaptively controlling an LED backlight, the driving current flowing through the LED backlight is controlled by a duty cycle of a PWM signal.

The aforementioned method for adaptively controlling the LED backlight, wherein when the analysis result shows that the brightness of the image data is lower than a preset value, the LED backlight is under-driven so that the light flowing through the LED The drive current of the diode backlight is less than the normal current.

The aforementioned method for adaptively controlling the LED backlight further includes: recording the overdrive time of the LED backlight; and estimating the overdrive time according to the duty cycle of the PWM signal and the recorded overdrive time The temperature of the LED backlight source; wherein, when the estimated temperature reaches a high temperature upper limit, the driving current is reduced.

The aforementioned method for adaptively controlling an LED backlight further includes a temperature sensing step for detecting the temperature of the LED backlight that is overdriven, wherein, when the detected temperature is higher than a preset temperature threshold value, reduce the drive current.

The aforementioned method for adaptively controlling an LED backlight source further includes using a processor to execute the following steps: accumulating the driving current within a period to form an accumulated driving current value; according to the accumulated driving current value and the period, to obtaining an average current value; and reducing the driving current when the average current value is greater than an upper limit; wherein, repeating the step of reducing the driving current until the average current value is less than the upper limit minus a hysteresis value.

The aforementioned method for adaptively controlling the LED backlight source further includes using a processor to perform the following steps: using a first counter for counting to generate a first cumulative driving current for a unit period; using a second counter Counting is used to generate the second cumulative driving current of the double unit period, wherein the first half of the double unit period coincides with the unit period; according to the second cumulative driving current and the double unit period, a average driving current; obtaining the difference between the second cumulative driving current and the first cumulative driving current as a new first cumulative driving current; and repeating the counting steps of the second counter to obtain a new double unit A new second cumulative driving current during the period, wherein the first half of the new double unit period coincides with the second half of the double unit period.

Compared with the prior art, the present invention has obvious advantages and beneficial effects. With the above-mentioned technical solutions, the system and method for adaptively controlling LED backlights of the present invention have at least the following advantages and beneficial effects: The system and method for adaptively controlling LED backlights of the present invention can be used to greatly enhance the dynamic contrast of displayed images Or/and save considerable energy.

To sum up, the present invention relates to a system and method for adaptively controlling LED backlight. It analyzes the brightness of the image data to be displayed on the display panel through the content analyzer. And the light emitting diode current controller is used to control the illumination of the light emitting diode backlight source through the light emitting diode driver according to the analysis result of the content analyzer. Wherein when the analysis result of the content analyzer shows that the brightness of the image data is higher than a preset value, the LED current controller over-drives the LED backlight, so that the driving current flowing through the LED backlight is greater than one. normal current. The present invention has significant progress in technology, and has obvious positive effects, and is a novel, progressive and practical new design.

The above description is only an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention, it can be implemented according to the contents of the description, and in order to make the above and other purposes, features and advantages of the present invention more obvious and understandable , the following preferred embodiments are specifically cited below, and are described in detail as follows in conjunction with the accompanying drawings.

Description of drawings

FIG. 1A is a schematic diagram of an LED backlight.

FIG. 1B is a graph showing the relationship between the luminous intensity of a light emitting diode and forward current.

FIG. 2 is a schematic diagram of a system for adaptively controlling an LED backlight according to a first embodiment of the present invention.

3A to FIG. 3C are diagrams illustrating the relationship between the normal dynamic range of the display panel, the dynamic range of the image content, and the dynamic range of the under-drive/over-drive display panel.

FIG. 4 is a schematic diagram of a part of the system for adaptively controlling the LED backlight according to the second embodiment of the present invention.

FIG. 5 is a schematic diagram of a system for adaptively controlling an LED backlight according to a third embodiment of the present invention.

FIG. 6A is a flowchart of a method for adaptively controlling an LED backlight according to a fourth embodiment of the present invention.

FIG. 6B is a time-varying graph illustrating the driving current of FIG. 6A.

FIG. 7A is a flowchart of a method for adaptively controlling an LED backlight according to a fifth embodiment of the present invention.

FIG. 7B is a time-varying graph illustrating the driving current of FIG. 7A.

FIG. 7C is a schematic diagram illustrating a duty cycle of a PWM signal obtained by using a sampling clock.

10: Light-emitting diode (LED) backlight 11: Display panel

12: Content Analyzer 13: Light Emitting Diode (LED) Current Controller

14: LED driver 141: Pulse width modulation (PWM) controller

142: Current limiter 15: Data adjustment unit

16: Temperature Estimation Unit 17: Overdrive Timer

18: Temperature sensor 21～25: Dynamic range

61~66: Steps 71~74: Steps

detailed description

In order to further explain the technical means and effects of the present invention to achieve the intended purpose of the invention, the specific implementation of the system and method for adaptable control of light-emitting diode backlight proposed according to the present invention will be described below in conjunction with the accompanying drawings and preferred embodiments. , structure, method, step, feature and effect thereof, detailed description is as follows.

The aforementioned and other technical contents, features and effects of the present invention will be clearly presented in the following detailed description of preferred embodiments with reference to the drawings. Through the description of the specific implementation, it should be possible to obtain a deeper and more specific understanding of the technical means and effects of the present invention to achieve the intended purpose, but the attached drawings are only for reference and description, not for the purpose of the present invention. be restricted.

FIG. 2 is a schematic diagram of a system for adaptively controlling a light emitting diode (LED) backlight according to a first embodiment of the present invention. The system of this embodiment is applicable to a display panel 11 (such as a liquid crystal display panel), which can be illuminated by an LED backlight.

In this embodiment, the content analyzer 12 analyzes the characteristics (such as brightness) of the image data to be displayed on the display panel 11 . The LED current controller 13 drives the LED backlight source 10 through the LED driver (or driving board) 14 according to the analysis result of the content analyzer 12 . Wherein, the LED driver 14 includes a pulse width modulation (PWM) controller 141 for determining a duty cycle (of the PWM signal), during which the LEDs of the LED backlight 10 are turned on to illuminate the display panel 11 . In this way, the PWM signal with a larger duty cycle allows more current to flow through the LEDs of the LED backlight 10 , thereby generating greater brightness (making them brighter). Conversely, a PWM signal with a smaller duty cycle allows less current to flow through the LEDs of the LED backlight 10, thereby producing less brightness (making it darker). The LED driver 14 may also include a current limiter 142, which limits the current entering the LED backlight 10 to an upper limit (for example, using a temporary register to set the upper limit), in order to avoid the LED backlight 10 An overheating condition has occurred.

In operation, when the analysis result of the content analyzer 12 shows that the brightness of the image data is low (that is, a dark image), the LED current controller 13 controls the pulse-width modulation (PWM) controller 141 so that the current flowing into the LED The current of the LEDs of the backlight 10 is lower than a normal current (eg, the current recommended by the LED manufacturer). In other words, the LED backlight 10 at this time is under-drive or operating in an under-drive mode. FIG. 3A is a graph illustrating the relationship between the normal dynamic range of the display panel 11 , the dynamic range of video content, and the dynamic range of the under-drive display panel 11 . As shown in FIG. 3A , the dynamic range 21 of the dark image only accounts for a small portion of the entire dynamic range 22 . By under-driving the LED backlight 10 , the under-drive dynamic range 23 of the display panel 11 is much smaller than the normal display dynamic range 24 , thereby saving considerable energy.

According to one of the characteristics of this embodiment, when the analysis result of the content analyzer 12 shows that the brightness of the image data is relatively high (that is, a bright image), the LED current controller 13 controls the pulse width modulation (PWM) controller 141, Make the current flowing into the LEDs of the LED backlight 10 higher than the normal current (for example, the current recommended by the LED manufacturer). In other words, the LED backlight 10 at this time is over-driven or operating in an over-drive mode. FIG. 3B is a graph illustrating the relationship between the normal dynamic range of the display panel 11 , the dynamic range of the image content, and the dynamic range of the over-drive display panel 11 . As shown in FIG. 3B , the dynamic range 21 of bright images occupies almost the entire dynamic range 22 . By over-driving the LED backlight 10, the dynamic range 25 of the over-drive display panel 11 is larger than the normal display dynamic range 24, thereby increasing the dynamic contrast of bright images.

Figure 3C is a diagram illustrating another overdrive embodiment of Figure 3B. Figure 3C uses less overdrive than Figure 3B. Thereby, the dynamic range 25 of the over-drive display panel 11 is substantially equal to the normal dynamic range of the display panel 11 . Therefore, the number of light emitting diodes can be reduced to reduce the cost.

The above-mentioned content analyzer 12 and LED current controller 13 can be set in the timing controller (T-con) of the imaging system, and the implementation method can use hardware, software or a combination thereof. The content analyzer 12 and the LED current controller 13 can also be disposed in a system-on-chip (SOC) processor, which is usually located before the timing controller (T-con). The system shown in this embodiment can also include a data adjustment unit 15, which is used to adjust or remap the dynamic range 21 (FIG. 3A/FIG. 3B/FIG. 3C) of the image to the entire dynamic range 22 before the image data is fed to the display panel 11. .

According to another feature of the present embodiment, some mechanisms are disclosed to protect the over-driven LED backlight 10 from overheating conditions. FIG. 4 is a schematic diagram of a part of the system for adaptively controlling the LED backlight according to the second embodiment of the present invention. In this embodiment, the temperature estimating unit 16 estimates the temperature of the overdriven LED backlight 10 according to the duty cycle of the PWM signal and the overdriven period recorded by the overdriven timer 17 . When the estimated temperature reaches the upper upper temperature limit, it indicates excessive heat generation, so the overdrive mode is temporarily turned off (ie, the normal current is restored), or the overdrive current is temporarily reduced.

FIG. 5 is a schematic diagram of a system for adaptively controlling an LED backlight according to a third embodiment of the present invention. In this embodiment, a temperature sensor 18 is used to detect the temperature of the over-driven LED backlight 10 . When the detected temperature is higher than the preset temperature threshold, the overdrive current can be temporarily reduced by reducing the duty cycle of the PWM signal.

FIG. 6A is a flow chart of a method for adaptively controlling an LED backlight according to a fourth embodiment of the present invention, and FIG. 6B is a diagram illustrating time variation of the driving current of FIG. 6A . In step 61, the drive current value is accumulated during the period Δt, and then divided by Δt to obtain the average current value Iavg. Since the driving current is generally proportional to the duty cycle of the PWM signal, this step can replace the driving current by processing the PWM signal. In step 62, the average current value Iavg is compared with an upper limit value Imax. If the average current value Iavg is not greater than the upper limit value Imax, it means that the LED backlight source 10 is not suspected of overheating, and then the process returns to step 61 to accumulate the driving current for the next Δt period to obtain another average current value Iavg, Wherein, the next Δt period is shifted for a period of time compared with the previous Δt period. If step 62 determines that the average current value Iavg is greater than the upper limit value Imax, then enter step 63 to reduce the driving current to prevent the LED backlight 10 from overheating. In particular, step 63 is to gradually reduce the driving current so that the viewer will not feel a sudden change of the displayed image. Next, step 64 (similar to step 61 ) performs accumulation of driving currents to obtain an average current value Iavg for the next Δt period. In step 65, the average current value Iavg is compared with the upper limit value Imax minus the hysteresis value Ih (ie, Imax−Ih). If the average current value Iavg is not less than the upper limit value Imax minus the hysteresis value Ih (that is, Imax-Ih), it means that the drive current has not dropped enough, so repeat steps 63 and 64 until the average current value Iavg is less than the upper limit value The hysteresis value Ih is subtracted from Imax (that is, Iavg<Imax−Ih), and the process enters step 66 to increase the driving current to enhance the dynamic contrast of the bright image. In particular, step 66 is to gradually increase the driving current so that the viewer will not feel a sudden change of the displayed image. After completing step 66, the flow returns to the initial step 61.

Since the accumulation of the drive current value requires a buffer or memory to store the drive current value during Δt, and the acquisition and comparison of the average drive current requires a comparison capability, the fourth embodiment (FIG. 6A/FIG. 6B) can be implemented in timing control device (T-con) or system-on-chip (SOC) processor to utilize its computing and storage resources. FIG. 7A is a flow chart of a method for adaptively controlling an LED backlight according to a fifth embodiment of the present invention, which does not require a buffer or a memory to store a driving current value. This embodiment can be implemented in the LED driver 14, which generally lacks computing and storage resources. FIG. 7B is a time-varying graph illustrating the driving current of FIG. 7A.

In step 71, the first counter (or variable) CNT1 is used to count the first cumulative drive current for the (small) unit period Δt, and in step 72, the second counter (or variable) CNT2 is used to count the double unit The second cumulative drive current during 2Δt. Wherein, the first half of the double unit period 2Δt overlaps with the unit period Δt. In step 73 , divide the second cumulative driving current CNT2 by twice the unit period 2Δt to obtain the average driving current. Next, in step 74, the difference between CNT2 and CNT1 is obtained as a new (updated) CNT1. The process goes back to step 72 to obtain a new (updated) second cumulative driving current CNT2 for a new double unit period 2Δt. Wherein, the first half of the new double unit period 2Δt overlaps with the second half of the old (original) double unit period 2Δt. In this embodiment, the duty cycle of the PWM signal can be processed instead of the driving current. As shown in FIG. 7C , FIG. 7C is a schematic diagram illustrating using a sampling clock to obtain a duty cycle of a PWM signal, wherein a sampling clock with a frequency higher than that of the PWM signal can be used to sample the PWM signal to obtain a duty cycle of the PWM signal.

The above description is only a preferred embodiment of the present invention, and does not limit the present invention in any form. Although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention. Anyone familiar with this field Those skilled in the art, without departing from the scope of the technical solution of the present invention, may use the method and technical content disclosed above to make some changes or modifications to equivalent embodiments with equivalent changes, but if they do not depart from the technical solution of the present invention, Any simple modifications, equivalent changes and modifications made to the above embodiments according to the technical essence of the present invention still fall within the scope of the technical solutions of the present invention.

Claims (14)

1. A system for adaptively controlling light-emitting diode backlights, characterized in that it comprises: A content analyzer for analyzing the brightness of image data to be displayed on a display panel; a light emitting diode driver, used to drive the light emitting diode backlight; an LED current controller, controlling the illumination of the LED backlight through the LED driver according to the analysis result of the content analyzer; and A current limiter, used to limit the driving current flowing through the LED backlight to an upper limit; Wherein, when the analysis result of the content analyzer shows that the brightness of the image data is higher than a preset value, the LED current controller over-drives the LED backlight so that the driving current flowing through the LED backlight is greater than a normal current. 2. The system for adaptively controlling LED backlight according to claim 1, wherein said LED driver comprises a pulse width modulation controller for determining a duty cycle of a pulse width modulation signal , wherein the PWM signal is coupled to control the driving current flowing through the LED backlight. 3. The system for adaptively controlling LED backlight according to claim 2, wherein when the analysis result of the content analyzer shows that the brightness of the image data is lower than a preset value, the LED current control The driver is insufficient to drive the LED backlight, so that the driving current flowing through the LED backlight is less than the normal current. 4. The system for adaptively controlling LED backlight according to claim 2, further comprising: an overdrive timer for recording the overdrive time of the LED backlight; and a temperature estimating unit, estimating the temperature of the overdriven LED backlight according to the duty cycle of the PWM signal and the recorded overdriving time; Wherein, when the estimated temperature reaches a high temperature upper limit, the driving current is reduced. 5. The system for adaptively controlling LED backlight according to claim 1, further comprising a temperature sensor for detecting the temperature of the LED backlight that is over-driven, wherein when detecting When the measured temperature is higher than a preset temperature threshold, the driving current is reduced. 6. The system for adaptively controlling LED backlight according to claim 1, further comprising a processor for performing the following steps: accumulating the driving current in a period to form an accumulated driving current value; obtaining an average current value according to the accumulated drive current value and the period; and reducing the drive current when the average current value is greater than an upper limit; Wherein, the step of reducing the driving current is repeatedly performed until the average current value is less than the upper limit value minus a hysteresis value. 7. The system for adaptively controlling LED backlight according to claim 1, further comprising a processor for performing the following steps: using a first counter for counting to generate a first accumulative driving current for a unit period; A second counter is used for counting to generate a second cumulative driving current of twice the unit period, wherein the first half of the double unit period coincides with the unit period; obtaining an average driving current according to the second accumulated driving current and the double unit period; obtaining the difference between the second cumulative driving current and the first cumulative driving current as a new first cumulative driving current; and The counting step of the second counter is repeated to obtain a new second accumulated driving current of a new double unit period, wherein the first half of the new double unit period coincides with the second half of the double unit period. 8. A method for adaptively controlling a light-emitting diode backlight source, characterized in that it comprises the following steps: analyzing brightness of image data to be displayed on a display panel to generate an analysis result; controlling the illumination of the LED backlight according to the analysis result to drive the LED backlight; and limiting the driving current flowing through the LED backlight to an upper limit; Wherein, when the analysis result shows that the brightness of the image data is higher than a preset value, the LED backlight is over-driven so that the driving current flowing through the LED backlight is greater than a normal current. 9. The method for adaptively controlling an LED backlight as claimed in claim 8, wherein the driving current flowing through the LED backlight is controlled by a duty cycle of a PWM signal. 10. The method for adaptively controlling an LED backlight according to claim 9, wherein when the analysis result shows that the brightness of the image data is lower than a preset value, it is not enough to drive the LED backlight, Make the driving current flowing through the LED backlight less than the normal current. 11. The method for adaptively controlling LED backlight according to claim 9, further comprising: record the overdrive time of the LED backlight; and Estimating the temperature of the overdriven LED backlight according to the duty cycle of the PWM signal and the recorded overdriving time; Wherein, when the estimated temperature reaches a high temperature upper limit, the driving current is reduced. 12. The method for adaptively controlling an LED backlight according to claim 8, further comprising a temperature sensing step for detecting the temperature of the LED backlight that is overdriven, wherein when detecting When the measured temperature is higher than a preset temperature threshold, the driving current is reduced. 13. The method for adaptively controlling LED backlight according to claim 8, further comprising using a processor to perform the following steps: accumulating the driving current in a period to form an accumulated driving current value; obtaining an average current value according to the accumulated drive current value and the period; and reducing the drive current when the average current value is greater than an upper limit; Wherein, the step of reducing the driving current is repeatedly performed until the average current value is less than the upper limit value minus a hysteresis value. 14. The method for adaptively controlling LED backlight according to claim 8, further comprising using a processor to perform the following steps: using a first counter for counting to generate a first accumulative driving current for a unit period; A second counter is used for counting to generate a second cumulative driving current of twice the unit period, wherein the first half of the double unit period coincides with the unit period; obtaining an average driving current according to the second accumulated driving current and the double unit period; obtaining the difference between the second cumulative driving current and the first cumulative driving current as a new first cumulative driving current; and The counting step of the second counter is repeated to obtain a new second accumulated driving current of a new double unit period, wherein the first half of the new double unit period coincides with the second half of the double unit period.